Blended families, those that have children from outside the marriage, are more and more common these days. Marriages can be "seconds" for one or both spouses. And sometimes they bring a child(ren) from a prior relationship or marriage into the mix. I myself was raised in a blended family. When my parents married in 1970 my father had a 7 year old son from his first marriage. Three years later, I joined the clan, putting 10 years in age between myself and my sibling. Other folks marry later in life, following the death of a spouse, and bring adult children to the marriage. What happens when that parent dies?
Unless a will and pre-planning has occurred, state statues will control the distribution of those assets. The new spouse may inherit everything. Or, the children from the first marriage may have inheritance rights to all or a share of the decedent's assets. What is important is for blended families to sit down, look at all of the assets, and ask "what do we want to happen when the first of us dies?". Once those answers are ironed out, put the plan into legal form with the assistance of an estate planning attorney in your state. And if the discussion is not going well, seek counsel of an attorney to help guide you through stalemates and pitfalls. Remember, if you do nothing, state statues will control....not you.
For more reading on planning for blended families, check out this article, with special attention to the book by Richard E. Barnes.